1. Field of the Invention
The present invention relates to an objective lens drive device which has an objective lens holder mounted on by a support shaft to be movable at least in the axial direction of the support shaft.
2. Description of the Prior Art
A conventional objective lens drive device of the biaxial drive type as shown in FIGS. 1 to 3 is known. Such objective lens drive device has a focusing control function for controlling the movement of the objective lens in the direction of the optical axis and also a tracking control function for controlling the movement of the objective lens in the direction perpendicular to the optical axis. This device will be described in more detail with reference to FIGS. 1 to 3.
An objective lens holder 1 consists of an integral assembly of a cylinder 1a and an end plate 1b. The upper end of a bearing 2 is inserted through the central portion of the lens holder 1 and is fixed thereto. The upper end of a support shaft 4 is slidably inserted in the bearing 2 while the lower end of the support shaft 4 is inserted in a substantially circular base 3. In other words, the objective lens holder 1 is supported by the support shaft 4 through the bearing 2 such that it is rotatable around the support shaft 4 and is slidable along the axial direction of the support shaft 4.
An objective lens 6 is held through a lens frame 5 at a position in a projection 1c of the end plate 1b of the objective lens holder 1, which is spaced apart from the axis of the support shaft 4. The objective lens 6 is supported such that its optical axis is parallel to the axis of the support shaft 4.
A focusing coil 7 is wound around a circumferential wall 1d of the objective lens holder 1 having the axis of the support shaft 4 as a central axis. Four substantially square tracking coils 8A to 8D are adhered in a layered manner on the outer surface of the focusing coil 7 at equal intervals along the circumferential direction. Opposing sides a and b of each tracking coil are kept parallel to the axis of the support shaft 4.
A pair of inner yokes 3b and 3c extend from an upper surface 3a of the base 3 and project inside the objective lens holder 1 such that the yokes 3b and 3c oppose each other with the support shaft 4 interposed therebetween. A pair of first notches 3d and 3e are formed at the sides of the support shaft 4, which is sandwiched between the pair of yokes 3b and 3c. A ring-shaped magnet 9 and a pair of connected outer yokes 10a and 10b are arranged outside the inner yokes 3b and 3c and extend the first notches 3d and 3e and outside the objective lens holder 1. The outer yokes 10a and 10b are connected to the upper side surface of the magnet 9. Second notches 10c and 10d are formed between the outer yokes 10a and 10b. The outer yokes 10a and 10b and the second notches 10c and 10d are arranged to be opposed to the inner yokes 3b and 3c and to the first notches 3d and 3e, respectively, through the circumferential wall 1d of the objective lens holder 1.
The circumferential wall 1d of the objective lens holder 1 is located such that the focusing coil 7 and the sides a of the tracking coils 8A to 8D are arranged in the magnetic gap of a magnetic circuit consisting of the inner yokes 3b and 3c, the outer yokes 10a and 10b, and the magnet 9, or a magnetic gap defined between the inner yokes 3b and 3c and the outer yokes 10a and 10b.
In the above-mentioned magnetic gap, a magnetic field is formed in the direction perpendicular to the circumferential wall 1d of the obejctive lens holder 1. Therefore, the objective lens holder 1 is electromagnetically driven in the focusing direction, that is, in the axial direction of the support shaft 4 by a current flowing in focusing coil 7 along the circumferential direction of the circumferential surface 1d, that is, a current flowing along the circumferential direction of a circle having the axis of the support shaft 4 as a center. At the same time, the objective lens holder 1 is also electromagnetically driven in the tracking direction, that is, in the pivoting direction around the axis of the support shaft 4, by currents flowing in the sides a of the tracking coils 8A to 8D in the same direction parallel with the axial direction of the support shaft 4.
A damper member 11 of rubber or the like is interposed between the objective lens holder 1 and the base 3. The damper member 11 serves as a neutral position holding mechanism which automatically returns the objective lens holder 1 to the neutral position when the objective lens holder 1 is released from being driven in the focusing or tracking direction.
If the objective lens holder 1 of the device described above is made thinner in the axial direction of the support shaft so as to render the overall device thinner, the distal ends of the inner yokes 3b and 3 arranged inwardly of the objective lens holder abut against the end plate of the objective lens holder. If the inner yokes are reduced in size so as to prevent this, the opposing areas between the inner yokes 3b and 3c and outer yokes 10a and 10b are reduced, and a sufficient magnetic force cannot be obtained. In other words, a sufficient driving force required for focusing or tracking cannot be obtained, resulting in a low sensitivity.